Ab initio complete active space self-consistent field and second order Moller-Plesset perturbation calculations have been performed to examine the capabilities of the second order non-linear optics (NLO) of 4-dimethylamino,4'-cyano,i-sila(or germa)stilbenes (i-DCSS, or i-DCSG) where the carbon at the position i of the parent 4-dimethylamino,4'-cyanostilbene (DCS) is replaced by more electro-positive silicon or germanium atom. 1-DCSS and 1-DCSG, which have not been synthesized yet, were found to have better NLO properties than DCS. (C) 2003 Elsevier Science B.V. All rights reserved.